This invention relates to a controlled-release product and to a process for its preparation and in particular, to a controlled-release fertilizer.
Nitrogen, phosphorus and potassium are the primary essential elements for plant growth, and commercially available fertilizers invariably comprise either or both an organic and an inorganic compound containing one or more of these elements in some form. Agronomically, most commercially available fertilizer materials are marked by one notably detrimental physical property -- high water solubility. Thus, treatment of plants with highly water soluble fertilizer materials, particularly nitrogen-containing, can be undesirable when used as such because the nutrient is readily solubilized in the soil solution resulting in luxury consumption and nutrient imbalances. If the release of nutrients were controlled to more closely match actual plant requirements, a number of advantages would accrue. Evaluated from the viewpoint of plant physiology, controlled-release fertilizers, as compared to many conventional fertilizers, maintain a more optimum concentration of nutrient in the root zone over a larger part of the plant growth period. Particularly in the case of turfgrass, controlled-release fertilizers applied in a single application provide a more even rate of growth than split applications of a soluble fertilizer at comparable rates over a growing season. Other advantages of controlled-release fertilizers are minimization of nutrient losses by runoff or volatilization, reduction of application costs through reduction in frequency of applications and prevention of vegetation burning or damage to seedlings.
Many attempts to control nutrient release to plants have been made with varying degrees of success. These efforts have focused on two broad areas: (1) development of polymeric or discrete chemical compounds that have limited water solubility, and (2) altering soluble compounds in some manner to retard their release in the soil solution, as by coatings or matrices which are water-insoluble. A number of patents have issued on the results of these efforts, many of which are directed to coatings for fertilizers. Such coatings have included single or multiple layers of natural occuring and synthetic polymers, waxes, inorganics including sulfur, diatomaceous earth, clays, glass asbestos, calcium carbonates and various combinations of the foregoing.
However, coatings must satisfy a demanding list of requirements. Coating materials should ideally be water soluble, inexpensive, easy to apply and not subject to blocking, melting, cracking or decomposition under a fairly wide range of environmental conditions encountered in storage and handling. In addition, coating materials should be substantially unreactive and effective at relatively low percentages by weight of the fnal product -- preferably 25% or less -- in order to provide the greatest fertilizer benefit and the lowest coating cost per unit weight of product. Known coatings are deficient in one or more of these requirements.